It is well known that foods located in vacuum or under reduced pressure can be preserved for a long time as the foods are prevented from chemical oxidation and decomposition during storage. The application of vacuum storage can be found in canned foods, retorted foods and the like. It is, however, difficult to carry out canning or retorting of foods with easy in the family since such a processing operation requires large-scaled plants.
To this end, there has been proposed a device for packing foods, which is operated by the steps of packing foods into a flexible bag such as polyethylene bag, degassing the bag and then sealing the same. Although such a device makes it possible to produce packed foods with ease, it is difficult to reduce the pressure in the bag since a degassing means merely draws the air from the bag. For this reason, it is required to use deformable bags made up of a flexible material such as polyethylene films. In addition, such a package occasionally makes it difficult to safely preserve the foods for a long time, since a large amount of air remains in the bag if the packed foods are of a complex configuration. Also, it is difficult to apply such a device to pack foods containing liquid.
To solve these problems, the present inventors have made a vacuum storage device on a trial basis, as shown in FIG. 7. The vacuum storage device comprises a base member 101, a semispherical housing 102 removably mounted on the base member 101 to form a storage chamber 103, and a pressure reducing device 104. The pressure reducing device 104 has a cylinder 105 and a piston 106 with a suction valve 107, which opens the cylinder 105 to the storage chamber 103 during the suction stroke, and a discharge valve 108, which opens the cylinder 105 to the open air during the discharging stroke. When preserving food in the device, the storage chamber 103 is reduced in pressure by manually operating the pump 104.
However, it has been found that such a vacuum storage device involves some problems to be solved. The greatest problem is operation of the pressure reducing device. If a cylinder chamber 109 between an upper cylinder wall 110 and piston 106 is communicated with the open air, the atmospheric pressure is applied to the whole upper surface of the piston 106 so that the suction stroke caused by a return spring 111 gets late in correspondence to decrease in the internal pressure of the storage chamber 103. If the chamber 109 is hermetically sealed from the atmosphere, it is required to apply a considerably increased force to the piston to cause the discharging stroke since the increase in capacity of the chamber 109 results in reduction in the internal pressure of the chamber 109.